1. Field of the Invention
The present invention relates to a power amplifier, and more particularly, to a power amplifier having a predistortion linearizer for improving a linearity of the power amplifier.
2. Background of the Related Art
Generally, a power amplifier has a non-linear property in which, as the number of input signals is increased, a gain of the amplifier is reduced and a phase thereof is delayed. Thus, in order to improve the non-linearity of the power amplifier, a wide variety of linearization techniques and algorithms have been developed. These include predistortion, envelope feedback, and feedforward. The predistortion method has been widely employed in mobile communication base stations since it has a simple structure and an excellent efficiency as compared to the feedfoward method. Additionally, unlike the envelope feedback method the predistortion method has no bandwidth limit.
FIG. 1 illustrates a related art power amplifier having a predistortion linearizer. As illustrated in FIG. 1, the related art power amplifier having a predistortion linearizer includes a power amplifier 14, a predistorter 12 for improving the non-linearity of the power amplifier 14, and a directional coupler 16 for sampling the output signal of the power amplifier 14. The device also includes a distortion component detection unit 20 for separating a distortion component from the sampled output signals and a power detector 22 determining the amount of the distortion components in the sample. An adaptive controller 24 is additionally provided for controlling the properties of the predistorter 12 according to the amount of the detected distortion components detected in the power detector 22.
The distortion detection unit 20 includes a mixer 20-1 for down-converting the output signal and a band pass filter (BPF) 20-2 for filtering the output of the mixer 20-1 and separating the distortion component from the output signal.
The operation of the related art power amplifier having a predistortion linearizer thusly constructed will now be described with reference to the accompanying drawings.
When an output signal is inputted through an input port 10, the predistorter 12 changes the gain and phase of the input signal to counteract the non-linear properties of the power amplifier 14. That is, the input signal is modified to minimize the non-linear property of the power amplifier 14. In addition, the input signal whose gain and phase are changed is amplified in the power amplifier 14, and then is outputted through an output port 18.
If the predistorter 12 operates normally, the output signal of the power amplifier 14 will have no distortion component. However, because it is so difficult to set the property of the predistorter 12 such that it has exactly the inverse of the non-linear property of the power amplifier 14, a distortion component appears in the output signal.
Therefore, in order to minimize the distortion component of the output signal, the adaptive controller 24 periodically checks the generated amount of distortion components detected in the power detector 22. The adaptive controller 24 then changes the properties of the predistorter 12 in order to minimize the generated amount of distortion components. Specifically, the distortion components are separated from the output signal, as shown in deviated crease line of FIG. 2. This is done by down-converting (RFout-LO) the output signal using the mixer 20-1, and thereafter passing the same through the BPF 20-2. Therefore, the power detector 22 determines the generated amount of the distortion component upon receipt of the distortion component separated from the BPF 20-2 and produces an output signal.
As described above, the related art power amplifier has various problems. For example, a distortion component is detected from the down-converted signal using the BPF 20-2. At this time, the output signal of the ideal BPF does not contain an input signal component to be amplified. Because the attenuation property of the filter is restricted, only the input signal component appears in the output of the BPF 20-2.
Additionally, in order for the detected distortion component to be used in the adaptive controller 22, the amplitude of the input component must be smaller than that of the distortion component by more than 15 dB. As a result, considering that the amplitude of the distortion component of a general power amplifier is smaller than that of the input component by approximately 30 dB, the BPF 20-2 has to attenuate the input signal component by more than 45 dB. Thus, in the related art, there is a problem that the attenuation property of the filter must be excellent in order to greatly improve the distortion property appearing in the output of the power amplifier.
Furthermore, the distortion component generated in the power amplifier 14 is mainly composed of third IMD (Intermodulation distortion). However, as illustrated in FIG. 2, there is a disadvantage that it is very difficult to filter the third IMD because it is directly adjacent to the original input component.
Moreover, the related art changes a center frequency of a local oscillation carrier (LO) and performs a down-conversion with respect to the changed center frequency of the local oscillation carrier (LO) in order to maintain the constant center frequency of the BPF 20-2. For this, accurate center frequency information of the input signal is required. However, in the case that the accurate frequency information is not acquired, additional modules for checking the frequency of the input signal are needed.
It is an object of the present invention to provide a power amplifier having a predistortion linearizer that substantially obviates problems due to disadvantages in the prior art.
It is another object of the present invention to provide a power amplifier having a predistortion linearizer capable of reducing the non-linear property of the power amplifier.
It is another object of the present invention to provide a distortion component detection circuit for a power amplifier capable of accurately detecting a distortion component irrespective of the frequency and properties of an input signal.
To achieve at least the above objects in whole or in parts, there is provided a power amplifier having a predistortion linearizer, which includes a predistorter for outputting a distortion compensating signal to the power amplifier; a delay line for delaying an input signal for a predetermined time and generating a reference signal; a distortion component detection circuit for detecting a distortion component by comparing the output signal of the power amplifier with the reference signal of the delay line; and an adaptive controller for controlling the properties of the predistorter in order to minimize the detected distortion component. The above distortion component detection circuit adjusts the output signal so that it has the same amplitude as the reference signal and the phase difference between the output signal and the reference signal is 180 degrees, and thereafter separates distortion components from the output signal of the power amplifier by vector operation of the output signal and the reference signal.
To further achieve the above objects in whole or in parts, there is provided a distortion component detection circuit having a vector modulator for controlling the gain and phase of an output signal; a branch line coupler for receiving an output signal and reference signal of the vector modulator and adding vector components of both signals; a 2-way combiner for combining outputs of the branch line coupler and outputting a distortion component; and a control signal generator for producing the difference in gain and phase by computing the output signal and reference signal of the vector modulator, the output signal of the branch line coupler, and the power of the distortion component and outputting the control voltage proportional to the difference in the outputted gain and phase to the vector modulator.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.